A can body of a drawn and ironed can (hereinafter referred to as the "DI can") which is a kind of the twopiece cans, is conventionally manufactured by means of a drawing and ironing forming (hereinafter referred to as the "DI forming") which comprises the steps of: drawing a circular sheet cut from a metal sheet by forcedly passing the circular sheet through a die with the use of a punch to form a cup, and then, redrawing and ironing the thus formed cup by forcedly and continuously passing the cup through a plurality of different dies with the use of another punch, thereby obtaining a can body having a thickness remarkably smaller than that of the circular sheet.
A can body of a drawn and redrawn can (hereinafter referred to as the "DRD can") which is also a kind of the two-piece cans, is conveniently manufactured by means of a drawing and redrawing forming (hereinafter referred to as the "DRD forming") which comprises the steps of: drawing a circular sheet cut from a metal sheet by forcedly passing the circular sheet through a die with the use of a punch to form a cup, and then, redrawing the thus formed cup by forcedly passing the cup through another die with the use of another punch, thereby obtaining a can body.
The two-piece can such as the DI can or the DRD can as described above is light in weight because of the reduced thickness as compared with a so-called threepiece can which comprises a top, a bottom, and a drum prepared by soldering or welding, and free from leakage of the content because of the absence of a joint in the drum. For these advantages, there is an increasing demand for two-piece cans, and the range of uses thereof is expected to expand.
As a metal sheet for the two-piece can, it is the usual practice to use an electrolytic tin-plated steel sheet or an aluminum sheet. The demand for the two-piece can made of the electrolytic tin-plated steel sheets is expected to grow since the electrolytic tin-plated steel sheet is lower in cost than the aluminum sheet. The weight of deposited tin of the electrolytic tin-plated steel sheet is relatively slight. However, under the influence of the recent cost increase of electric power and tin, the increase in the manufacturing cost of the electrolytic tin-plated steel sheets and the two-piece cans made of the electrolytic tin-plated steel sheets is inevitable. There is therefore a strong demand for reducing the manufacturing cost of the electrolytic tin-plated steel sheets.
The reduction ratio of the thickness of the side wall of the can body of the DI can is so large as about 70% under the effect of ironing. It is therefore necessary to previously impart lubricity to the surface of the metal sheet. The tin plating layer of an electrolytic tin-plated steel sheet provides an excellent lubricity to the steel sheet. However, if the weight of the deposited tin plating layer is reduced with a view to reducing the manufacturing cost of the electrolytic tin-plated steel sheets, the following problems are caused in a DI can:
(1) The smaller weight of the deposited tin plating layer results in a lower lubricity which in turn leads to a lower DI formability. As a result, during the passage of the can body through the dies in ironing, a sticking of the can body to the dies or a galling of the can body may occur under the effect of friction heat. Occurrence of such sticking or galling deteriorates corrosion resistance of the DI can.
(2) Rust tends to occur on the surface of the can body of the DI can after removing grease such as an external lubricant deposited onto the surface of the can body of the DI can during ironing.
As methods for solving the above-mentioned problems, the following methods are known:
(1) A method for manufacturing a can body of the DI can, disclosed in Japanese Patent Provisional Publication No. 51-63,787 dated June 2, 1976, which comprises:
applying a precoating comprising any one of epoxyphenolic resin, epoxy-ureaformaldehyde resin and vinyl resin, or a precoating comprising any one of the abovementioned resins and an internal lubricant, onto at least one surface of a metal sheet to form a precoating film thereon, then partially curing the precoating film, and then subjecting the metal sheet having such a precoating film to the DI forming, thereby obtaining a can body of the DI can (hereinafter referred to as the "prior art 1").
(2) A method for manufacturing a can body of the DI can, disclosed in Japanese Patent Publication No. 60-4,753 dated Feb. 6, 1985, which comprises:
applying a precoating comprising epoxy-phenolic resin or a precoating comprising epoxy-phenolic resin and an internal lubricant onto the both surfaces of a surface-treated steel sheet to form a precoating film thereon, then partially curing the precoating film, then subjecting the surface-treated steel sheet having such a precoating film to the DI forming, and then completely curing the precoating film, thereby obtaining a can body of the DI can (hereinafter referred to as the "prior art 2").
The above-mentioned prior art 1 involves the following problems: because the precoating film is cured only partially, a precipitate from the precoating film not only degrades the flavor and perfume of the content in the can, but also leads to a lower corrosion resistance of the DI can after painting.
The above-mentioned prior art 2 involves the following problems: curing of the precoating film accomplished in two steps requires more complicated manufacturing processes and results in a higher manufacturing cost of the DI can.
On the other hand, a DRD can involves the problem in that, during the DRD forming of a metal sheet having a precoating film, the precoating film is damaged, and as a result, the inner surface of the can is easily corroded by the content in the can.
Under such circumstances, there is a strong demand for the development of a precoating for a two-piece can, which permits formation, on at least one surface of a metal sheet, of its film imparting excellent DI formability, DRD formability and corrosion resistance to the metal sheet and free from damage during the DI forming or the DRD forming of the metal sheet, but such a precoating for a two-piece can has not as yet been proposed.